I'm going for 16:9. 4:3 has always annoyed me. I remember friends Galaxy phones doing 16:9 pix and I always preferred that when viewed on my MacBook.
Heck, I can't stand 4:3 on any TV/film to the point that I can't wait and sadly, one or two indie films are trying to be clever and shoot in 4:3 which means i totally bypass the film. The new skateboarding film Mid90s is shot in 4:3 and even though I'd like to see the film, I can't stand the idea of watching an entire film in that square picture format.
You should really use 4:3 and crop after to prevent losing parts of the image you might want. 16:9 is just cropping to begin with. With the new ultrawide preview on the sides of the frame it’s harder to see what’s available in 16:9 mode as well.